Fault zones are hypothesized to play a major role in focusing fluid expulsion from accretionary complexes. A small number of previous investigations have examined the hydrogeologic properties of these fault zones using borehole packer or submersible-based slug and constant rate flow tests. While these single-well experiments yielded the first data on the permeability of active fault zones, they could only investigate a small area around the borehole and could not ensure reliable determination of formation storage properties. Recent data from an unplanned experiment in the decollement zone of the Barbados accretionary prism demonstrate the potential for multiple-well hydrogeologic tests. Pressure data from a sealed borehole show a clear signal of drilling activity ~45 m away. Analysis of the pressure response suggests that decollement zone permeability lies between 1.0 and 1.2¿10-14 m2. This permeability is ~2--4 orders of magnitude greater than overlying and underlying sediment and thus appears sufficient to focus fluid flow along the decollement zone. This inadvertent test evaluated fault zone hydrogeologic parameters over a larger radius and longer timescale and at pressures closer to in situ than previous single-well tests. ¿ 2000 American Geophysical Union